1. Field of the Invention
The present invention relates to a refractive optical unit having a plurality of lens groups and a reflective optical element that reflects a light ray incoming from a subject along a first optical axis into a direction along a second optical axis intersecting with the first optical axis, the light ray traveling along the second optical axis to form an image of the subject on a image plane.
2. Description of Related Art
Photographic devices include those that have a refractive optical system capable of reflecting a light ray incoming along the first optical axis from a subject into a direction along a second optical axis extending orthogonal to the first optical axis and capable of forming an image on an imaging element. The use of such a refractive optical system makes it possible to reduce the thickness of the camera body.
In order to reduce the thickness by drawing the maximal advantage of the refractive optical system, it is preferable to reduce the number of optical members having the first optical axis and to arrange a lens group, like a zoom lens, needed to move in the optical-axis direction, on the light-exit surface side of a prism (reflective optical element) as much as possible (see JP-A-2005-37490, for example). This eliminates the necessity of arranging in the first optical axis a zoom lens to move in the direction of the optical axis thereof, thus obtaining an advantage of using a refractive optical system to a maximal extent. Many proposals on techniques for assembling a refractive optical system, which provides advantages in achieving the thickness reduction, are made (JP-A-2006-217474, for example).
Meanwhile, by determining at what degree the zoom magnification is taken, determining a body height dimension from the travel of the zoom lens to achieve its zoom magnification and then pack the zoom lens and the driver within that dimension, size reduction is available in height in addition to the thickness reduction.
FIGS. 3A and 3B show an example of a refractive optical unit reduced in size in addition to thickness reduction.
FIG. 3A shows a view of a refractive optical unit 10 as viewed from front while FIG. 3B shows a sectional view of the refractive optical system 10 as viewed in a direction A-A of FIG. 3A. In FIGS. 3A and 3B, there is shown a refractive optical system having four lens groups 11-14 including a prism 111 serving as a reflective optical element that reflects light incoming along a first optical axis OP1 from a subject into a direction of a second optical axis OP2 orthogonal to the first optical axis OP1.
In order to reduce the thickness, this example arranges an objective lens 110 only of the first lens group on the first optical axis OP1, thereby arranging the remaining lens 112, including the prism 111, of the first lens group and the second to fourth lens groups 12-14 in positions following the prism 111. Meanwhile, in order to reduce the height, dense mounting of: a first zoom driver ZM1 including a lead screw LS1 and motor M1 to move the second lens group 12 along the second optical-axis direction; and a second zoom driver ZM2 including a lead screw LS2 and motor M2 to move the fourth lens group 14 along the second optical-axis is done within the height.
Referring to FIGS. 3A and 3B, explanation is first made briefly as to in what way a light ray from a subject is guided up to an imaging element 15 shown lower in FIGS. 3A and 3B.
At first, the light ray entering from the subject through the objective lens 110 of the first lens group 11 reflects upon the prism 111 toward the second optical axis OP2 orthogonal to the first optical axis OP1. Furthermore, the light ray reflected toward the second optical axis OP2 travels through the remaining lens 111 of the first lens group 11 and through the second, third and fourth lens groups 12, 13 and 14 to reach the imaging element 15. In this example, each of the second and fourth lens groups 12 and 14 of the four lens groups 11-14 is a zoom lens.
In FIGS. 3A and 3B, there is shown the state that each of the second and fourth lens groups 12, 14 which are the zoom lenses is in the telephoto position by being driven by first and second zoom drivers ZM1, ZM2.
As described above, the height dimension is determined from the traveling range of the zoom lens where a zoom magnification is achieved, to thereby pack within the height dimension the second and fourth lens groups 12, 14 as well as the drivers ZM1, ZM2 for driving them. For this reason, in the telephoto position in FIGS. 3A and 3B, the second and fourth lens groups 12, 14 are extremely close to the third lens group 13. In the wide-angle position, the second and fourth lens groups 12, 14 are extended fully to the height dimension of the camera-body. This enables dense mounting in the body.
Meanwhile, in the example of FIGS. 3A and 3B, upon densely assembling the refractive optical system in the housing in this manner, the first lens group with a prism is assembled in the housing by use of the technique of Japanese Patent Application No. 2005-134077 or the like to thereby provide a linearity as to the first optical axis. Furthermore, by use of the technique described in JP-A-5-188259 or the like, the second to fourth lens groups are assembled on one guide rod to thereby provide a linearity as to the second optical axis, followed by assembling the unit entirety. By doing so, high accuracy is obtainable as to both the first and second optical axes.
Furthermore, in FIGS. 3A and 3B example, although not shown, by continuously urging the lens carrier held with the second and fourth lens groups in one direction by use of the technique of JP-A-2006-91408 or the like, the second and fourth lens groups are aligned in positions toward the guide rod, thereby preventing the unsteadiness as caused due to the manufacture tolerances on both the guide rod and the carrier.
However, because the third lens group as a fixed lens can be accurately assembled onto the guide rod, there possibly encounters a deviation occurring between the second and fourth lens groups aligned toward the guide rod and the third lens group at their lens centers.